The present invention relates generally to the conversion of a standard diaphragm electrolytic cell which is being used for chlorine and caustic (sodium hydroxide) production, to an electrolytic cell having a membrane for the same type of chemical production with the inherent advantageous characteristics of a membrane electrolytic cell. More particularly the present invention relates to a method for forming a membrane over a standard diaphragm electrolytic cell cathode by vacuum forming a matting material onto the foraminous electrode and subsequently applying a membrane material over top of the matting material which is fused into a thin and uniform substantially hydraulically impermeable film. Such a method will allow manufacturers having standard diaphragm electrolytic cell equipment in current use to convert that equipment to membrane electrolytic cells with a smaller capital expenditure to yield a savings in the operational costs associated with the use of membrane electrolytic cells.
Electrochemical methods of manufacture are becoming ever increasingly important to the chemical industry due to their greater ecological acceptability, potential for energy conservation, and the resultant cost reductions possible. Therefore a great deal of research and development effort is being applied to the electrochemical processes and the hardware for these processes. From this effort has come technological advances such as the dimensionally stable anode and various coating compositions therefor which permit ever narrowing gaps between the electrodes, such that the electrolytic cell has become more efficient for use in electrochemical processes. Also the hydraulically impermeable membrane has added a great deal to the potential use of electrolytic cells in terms of the selective migration of various ions across the membrane surface so as to exclude contaminants from the resultant product thereby eliminating some costly purification and concentration steps of processing.
One significant commercial possibility for these advances in electrolytic cells would be in chlorine and caustic production. Chlorine and caustic are essential and large volume commodities which are basic chemicals required by all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkali metal chlorides, with a major proportion of such production coming from diaphragm-type electrolytic cells. In the diaphragm cell process, brine (sodium chloride solution) is fed continuously into the anode compartment and flows through the diaphragm usually made of asbestos, backed by the cathode. To minimize back migraton of the hydroxide ions, the flow is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused alkali metal chloride present. The hydrogen ions are discharged from the solution at the cathode in the form of molecular hydrogen gas. The cathode solution, containing caustic, unreacted sodium chlorde, and other impurities, generally has been concentrated and purified later to obtain a marketable sodium hydroxide commodity and a sodium chloride which can be reused in a chlorine and a caustic electrolytic cell for further production of sodium hydroxide.
The dimensionally stable anode is today being used by a large number of chlorine and caustic producers but the extensive commercial use of the hydraulically impermeable membrane has been at least in part militated against by the substantial capital cost involved in converting from disphragm electrolytic cells to membrance electrolytic cells. This is caused by the difficulty in placing a more or less planar membrane onto the cathode assembly which is generally a three dimensional assembly onto which the asbestos diaphragm is placed by vacuum forming from a slurry. The diaphragm has been improved by adding to the slurry from which the diaphragm is deposited onto the cathode assembly, a polymeric material to act as a binding substance so as to improve the chemical resistivities of the diaphragm material. This dimensionally stable polymer modified diaphragm though is not a hydraulically impermeable membrane. Another approach has been to form a membrane on the electrode surface itself. The probelm with this approach is that most of the cathode assemblies in current use are foraminous in nature and such porosity makes it very difficult to deposit membrane material to form a film directly onto the cathode assembly.